water and ice on the Moon

The Moon as seen through a small telescope from Earth.
The dark patches are called maria from the Latin mares meaning
"seas." We know now that these regions are not water-filled but are
basins containing dark-colored basalt – vast craters gouged
out by impacting asteroids, billions of years ago, which were subsequently
flooded with lave.

Craters at and around the Moon's north pole. Those
circled in red were formed relatively recently. Those indicated by
green arrows are believed, from measurements made by the Mini-SAR
instrument of the Chandrayaan-1 spacecraft, to contain water-ice.

Historical introduction

For a long time, people thought that the dark patches we can see on the
Moon with our unaided eyes might be bodies of water. In fact we still call
these features maria from the Latin word mares
meaning "seas." The Greek historian Plutarch
was among those in the ancient world who expressed this view in his writings.

Much later, in his short but influential treatise Siderius Nunctius
(Starry Messenger), Galileo wrote: " ...
[the] brighter part [of the Moon] most fitly represents its land surface,
but its darker part the watery surface ..." Later, he came to doubt this
interpretation, but others, including John Wilkins,
Johannes Hevelius, Johann Schröter,
and William Herschel kept the idea of
a maritime Moon alive for the next two centuries.

Not surprisingly, the belief that there were great bodies of water on the
Moon was accompanied by the belief that there might also be life there (see
life on the Moon), including intelligent
life.

But the notion of lunar water fell from favor as it became clear that the
Moon lacks any kind of substantial atmosphere. Physics is very clear on
the fact that liquid water can't survive on a surface where the atmospheric
pressure is extremely low or zero.

Ice on the Moon

But what is true of liquid water isn't necessarily true of solid water –
ordinary ice. The problem for ice on the Moon is that most of the Moon's
surface experiences wide swings in temperature, from very cold to very hot
depending on whether the Sun is shining or not. The average surface temperature
during the day is 107°C (with a maximum of 123°C) – well above
the boiling point of water; at night it dips to an average, over the whole
Moon, of -153°C (and a minimum of -233°C). During a complete lunar
day, the Sun's rays illuminate most of the Moon's surface making it impossible
for any ice on or near the surface to survive. However, there are many craters
at or near the north and south lunar poles whose depths lie in permanent
shadow. Any ice that had been deposited in such craters, long ago, could
in theory still be there today.

To test this possibility, several recent lunar orbiting probes have been
equipped with instruments that can detect, in various ways, the presence
of water-ice on the Moon. The first data to suggest that there is ice at
the Moon's poles came from the Clementine
probe, in 1994. Confirmation came in March 1998 from observations made by
Lunar Prospector1 which led
to an initial conservative estimate of 300 million tons of ice. The instrument
aboard Lunar Prospector used for this determination could detect the presence
of water-ice (or, more precisely, hydrogen) to a depth of 0.5 meter. Since
lunar soil has been turned over by meteorite impacts during the past couple
of billion years to a depth of about 2 meters, water could theoretically
be present to this depth. However, if this water exists as crystals mixed
in with the lunar regolith, larger amounts of pure water ice deposits could
potentially exist at much greater depths.

NASA's mini-SAR instrument carried on India's Chandrayaan lunar orbiting
spacecraft in 2009 provided further information on the lunar ice. Mini-SAR,
a lightweight, synthetic aperture radar, found ice in more than 40 small
craters. These craters range in size from 2 to 15 km (1 to 9 miles) in diameter.
Although the total amount of ice depends on its thickness in each crater,
it is estimated there could be at least 600 million metric tons of water
ice.

The lunar ice is thought to have come from comets
which collided with the Moon in the remote past. Although most of the water
deposited on the lunar surface in this way would have been vaporized long
ago by the daytime heat of the Sun, in small, permanently shaded regions
at the poles it has evidently survived. Its presence is further evidence
that water, frozen or otherwise, is a common commodity on many worlds, and
this, in turn, raises the prospects for extraterrestrial life. The availability
of water on the Moon is also a major boost to any plans for establishing
a self-sustaining lunar colony.